1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic recording medium or a thermomagnetic or optical-magnetic recording medium used in magnetic disk units or the like, and to an information recording and reproducing apparatus using these recording media.
2. Background Art
In response to the increase in capacity of magnetic recording apparatus in recent years, efforts are being made to also increase the recording density of magnetic recording media. As the recording density increases, the single recording-bit size decreases, resulting in a pronounced superparamagnetic effect in which the magnetization on the magnetic recording film on the medium becomes thermally destabilized. As an index of the superparamagnetic effect, KuV/KT>40 is often employed, where Ku is uniaxial anisotropy constant, V is the volume of a single magnetic particle, K is the Boltzmann constant, and T is temperature. From the aforementioned expression, it is seen that, if the recorded magnetization is to be stabilized against the superparamagnetic effect, either the volume of the magnetic particle should be increased, or a material with a large uniaxial anisotropy constant (Ku) should be used in the recording film.
The current media are continuous media where the individual recording bits are made up of a number of magnetic particles. Accordingly, the recording film is required to employ a magnetic material with a large uniaxial anisotropy constant (Ku) value to achieve higher recording densities, from the viewpoint of preventing superparamagnetic effect. This necessarily requires an increase in the recording magnetic field intensity. Thus, the designing and manufacturing of a recording head capable of providing a large recording magnetic field intensity poses a challenge in the development of recording heads.
On the other hand, in patterned media where the individual recording bits are made up of single magnetic particles, because the volume (V) occupied by a single particle is large, there is no need to use a material with large uniaxial anisotropy constant (Ku) values for preventing superparamagnetic effect. As a result, the patterned media can record with a smaller magnetic field intensity than in the case of the aforementioned continuous media. The patterned media method also has the advantage that it does not produce noise due to disturbance in magnetization in the bit transition region. Therefore, the patterned media are expected to provide a promising method for high-density magnetic recording media in the future in combination with the perpendicular recording method, which is capable of achieving higher recording densities than the longitudinal recording method.
In the patterned media method, because the individual recording bits are made up of single magnetic particles, adjacent recording bits, namely, the particles, must be magnetically disconnected. FIG. 1 shows a schematic representation of a patterned medium. In the figure, numeral 1 designates a magnetic layer, 2 designates a micropattern of a recording bit formed by processing the magnetic layer, 3 designates an intermediate layer, 4 designates a soft magnetic layer, and 5 designates a substrate.
In the conventional patterned media, individual recording bits are formed by microfabrication, as shown in FIG. 1. FIG. 2 shows a typical method of microfabrication. As shown in FIG. 2A, a resist layer 6 is formed on a magnetic layer 1, and further a resist pattern 7 with concavities and convexities is formed by lithography. Then, as shown in FIG. 2B, using the resist pattern as a mask, the magnetic layer 1 is cut by a focused ion beam (FIB) using Ga ion 8. The resist layer is thereafter removed, thereby preparing a recording bit 2 as shown in FIG. 2C. The space between the recording bit 2 and the adjacent recording bit 2 may be filled with a non-magnetic layer 9 after the cutting process and then made flat, as shown in FIG. 2D. In another example of pattern formation, an imprint process as shown in FIG. 3 is used. In the imprint process, a SiN substrate is processed by electron beam lithography or the like to prepare a pattern mold 10. The pattern mold 10 is pressed against the resist layer 6 as shown in FIG. 3A to transfer the pattern 7 on the resist film as shown in FIG. 3B. Thereafter, as shown in FIG. 3C, the magnetic layer is cut by reactive ion etching (RIE), thereby preparing the recording bit 2 as shown in FIG. 3D. As a RIE gas 11, carbonyl gas is often used. The imprint process is disclosed in Non-patent Documents 1 and 2.
In a method disclosed in JP Patent Publication (Kokai) No. 2002-359138 A, a ferromagnetic layer formed on a substrate is selectively masked and then exposed to a reactive gas containing halogen, whereby an exposed portion and the underlayer are chemically altered into a non-magnetic ferromagnetic region by chemical reaction. In this example, the mask utilizes the self-organizing phenomenon of a block copolymer comprising two types of polymers with different dry etch resistance. After the self-organization, the polymer at portions with low dry etch resistance are removed during etching, and the lower magnetic layer is chemically altered by the etch gas. At portions with high dry etch resistance, the polymer remains even during etching, such that the lower magnetic layer does not become altered and its magnetic characteristics remain good. Thus, magnetic recoding can be performed at these portions. In JP Patent Publication (Kokai) No. 2003-151127 A, nanodots are arranged on the substrate using an ion beam or the likes, and a layer of magnetic material is formed such that the spaces (wells) between the nanodots are filled by the magnetic material. The material is thereafter removed such that the regularly arranged structure of the wells filled with the magnetic material can be exposed, thereby preparing a patterned magnetic recording medium. JP Patent Publication (Kokai) No. 2003-218346 A discloses a method of forming a fine pattern using a nano-particle. In this method, nano-particles arranged on a substrate are etched using a mask to form nanopores (opening), in which various materials including a magnetic material can be filled so as to produce a variety of kinds of devices.
As described above, when preparing a patterned medium, in which individual recording bits are formed by single magnetic particles, the magnetic layer is formed into a desired shape by microfabrication. In addition, a discrete track medium is known in which grooves are formed between recording tracks by microfabrication on a continuous medium in which the magnetic layer has been formed by the conventional sputtering method. FIG. 4 schematically shows a discrete track medium. As shown, in the discrete track medium, a groove 13 is cut between recording tracks 12. Numeral 14 indicates the direction across the tracks. Thus, in the discrete track medium, because the recording tracks are physically separated from the adjacent tracks, cross-talk during the recording or writing with a read head or a write head can be reduced as compared with the current continuous media, thereby advantageously improving the SN ratio.
Non-Patent Document 1: “Fabrication of perpendicular patterned media by nano-imprint method”: Digest of the 25th Annual Conference on Magnetics in Japan (2001), p. 240
Non-Patent Document 2: “MFM analysis of perpendicular patterned media with no magnetic material”: Digest of the 25th Annual Conference on Magnetics in Japan (2001), p. 22
Patent Document 1: JP Patent Publication (Kokai) No. 2002-359138 A
Patent Document 2: JP Patent Publication (Kokai) No. 2003-151127 A
Patent Document 3: JP Patent Publication (Kokai) No. 2003-218346 A